1. Field of the Invention
The present invention relates to a digital system for the generation of control signals for laser modulation multi-channel circuits for oscillating mirror projectors of scanned images.
It relates, among others, to the generation of moving images and to their projection as wide-angle pictures. This technique is used for example, in the simulation of the visual environment of pilots and their training for aerial combat.
This training is done in large-diameter spheres. The internal surface of these spheres forms a wide-angle reflecting screen, and there are reproductions of fighter aircraft cockpits at the center of these spheres.
2. Description of the Prior Art
In the prior art, images of targets are obtained by projection systems placed above and behind the pilot.
These systems consist of "light valve" or other type image generators which have low luminous efficiency and give a halo around the image itself of the target.
This halo is especially detrimental because, although it has low luminosity, the pilot easily distinguishes it first. This enables him to see the target located at its center without actually having to identify it. His training is thus distorted because he is unable, in these conditions, to exercise and demonstrate his attentiveness and qualities of observation which are indispensable for aerial combat.
It has been pointed out, however, that these image generators could be replaced by laser image projection systems which do not have these advantages and which, furthermore, are less costly and less bulky, and can be placed on either side of the cockpit in order to completely clear the pilot's field of visibility.
These projection systems comprise, among others, one or more laser sources, laser modulation circuits controlled by video signals of the image to be projected and two oscillating mirrors mounted, for example, on pencil-shaped galvanometers capable of deflecting the laser beam along two orthogonal axes so as to give a scanned image.
The requirement that the target image should be presented in an angular field of about ten degrees, corresponding substantially to an aircraft at 300 meters, and the need for adequate definition despite the as yet limited efficiency of galvanometers, mean that the projection of laser scanned images encounters two difficulties: these are, firstly, the non-linearity of the line galvanometer oscillations and secondly, a deterioration of the image, due to intermodulation given by the multi-channel laser modulator. The video lines have to be scanned on several lines simultaneously.
For to satisfy the needs of the human eye and those of realistic simulation, the definition of the target image should be at least 256 lines of 256 pixels.
Furthermore, since the target images are logically produced from parent images meeting television standards, their projection should be done at the same rate, in two frames of a maximum duration of 20 ms.
Furthermore, present technology does not provide for any galvanometer other than one having a maximum cut-off frequency in the range of 7000 Hz.
Thus the frame galvanometer, which oscillates at a fundamental frequency below 100 Hz, raises no problem, but the line galvanometer has to oscillate at a frequency of 256/40.10.sup.-3, namely 6400 Hz (256 pixels per line and, 2.times.20 ms which is very high).
It will be incapable of tracking the saw-toothed control signal harmonics.
The frequency of this signal should therefore be reduced. This is acceptable only if several video lines are projected simultaneously at each oscillation.